Covariant MaybePromise<T>
Covariant MaybePromise<T>

Since Promise<T> is Covariant, this should also be. Solves flow errors when trying to introduce MaybePromise<T> into more places within the executor.

jjergus/graphql-js,graphql/graphql-js,enaqx/graphql-js,jjergus/graphql-js,graphql/graphql-js,enaqx/graphql-js,graphql/graphql-js,enaqx/graphql-js